You stub your toe on the bedpost. Before your brain even registers the pain, a word explodes from your mouth — sharp, loud and oddly satisfying. Far from being a simple slip in manners, swearing is a reflex rooted deep in the structure of the human body, drawing on networks in the brain and autonomic nervous system that evolved to help us survive pain and shock. Research shows that a well-placed expletive can dull pain, regulate the heart and help the body recover from stress. The occasional outburst, it seems, isn’t a moral failure — it’s a protective reflex wired...